(a) Field of the Invention
The present invention relates to a liquid crystal display, and in particular, to a gate pulse width modulation method of a liquid crystal display.
(b) Description of Related Art
A liquid crystal display (LCD) includes an upper panel including a common electrode and a plurality of color filters and coated with an alignment layer, a lower layer including a plurality of pixel electrodes and thin film transistors (TFTs) and coated with an alignment layer, and a liquid crystal (LC) layer filled in a gap between the upper panel and the lower panel. The LCD generates electric fields in the LC layer by applying respective voltages to the pixel electrodes and the common electrode. The orientations of the LC molecules in the LC layer, which determine polarization of light passing through the LC layer, vary depending on the field strength. A polarizer or a pair of a polarizer and an analyzer convert the light polarization into the transmittance of the light. Accordingly, the LCD displays desired images by controlling the voltages applied to the pixel electrodes and the common electrode.
In circuital view, the LCD includes a plurality of pixels arranged in a matrix and a plurality of signal lines connected to the pixels such as gate lines and data lines. Each pixel includes a LC capacitor including a pixel electrode, a common electrode, and a liquid crystal disposed between the pixel electrode and the common electrode, a switching element such as a TFT connected between the signal lines and the LC capacitor, and a storage capacitor connected to the switching element in parallel to the LC capacitor. The switching element selectively transmits data voltages from a data line connected thereto in response to the gate signal from a gate line connected thereto. The gate signal includes a gate-on voltage for turning on the switching element and a gate-off voltage for turning off the switching element. The LC capacitor is charged for the duration of the gate-on voltage.
In the meantime, since long-term application of a unidirectional electric field deteriorates the characteristics of the LC layer, the voltages applied to the pixel electrodes (referred to as “data voltages” hereinafter) are periodically reversed with respect to the voltage applied to the common electrode (referred to as “common voltage” hereinafter) such that the field direction applied to the LC molecules is periodically reversed. This technique is called “inversion.”
There are several types of the inversion such as one-dot inversion and double-dot inversion. The one-dot inversion reverses the polarity every row and every column, while the double-dot inversion reverses the polarity every two rows and every two columns.
When an LCD is subject to the double-dot inversion, the charging time of a pixel having a polarity opposite that of a previous pixel located along a column direction is longer than the charging time of a pixel having the same polarity as a previous pixel located along a column direction. If the duration of the gate-on voltage for the former pixel is short, the data voltage is not fully charged in the pixel. Therefore, there is an unbalance in charged voltages between the former pixel and the latter pixel. Such an unbalance causes defects on an LCD screen such as transverse stripes. The problem is particularly severe for a large, high resolution LCD since the duration of the gate-on voltage depends on the size and the resolution of the LCD and it is very short for the large, high resolution LCD.